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MEMOIR 







ON 



Foundations in Compeessible Soils, 



EXPERIMENTAL TESTS OF PILE-DRIVING, 



AND 



FORMULA FOR RESISTANCE DEDUCED THEREFROM. 



COMPILED BY 



RIGHT) DELAFIELD, Bv't Maj. Gen'l, 

Corps of Engineers, U, S. Army, Member of the L.-H. Board, 



WASHINGTON, D. C., DECEMBER 1, 1868. 
PUBLISHED BY ORDER OF THE LIGHT-HOUSE BOARD. 




WASHINGTON: 

GOVERNMENT PRINTING OFFIC 

1881. 






MEMOIR 



ON 



Foundations in Compressible Soils, 



WITH 



EXPERIMENTAL TESTS OF PEE-DRIYING, 



FOKMULA FOE RESISTANCE DEDUCED THEREFROM. 



COMPILED BY 



RICH'D DELAFIELD, Bv't Maj. Gen'l, 

Corps of Engineers, TL S. Army, Member of the L.-H, Board, 



WASHINGTON, D. C, DECEMBEE 1, 1868. 
PUBLISHED BY ORDER OF THE LIGHT-HOUSE BOAI 

£5£2p v 






c 



WASHINGTON: 

GOVERNMENT PRINTING OFFICE. 

1881. 




17. Annual Reports of the Chief Engineer, accompanying the President's Message to 

Congress, from 1824 to 1868. On Foundations and their Construction for the 
Forts in Louisiana, on the Eip-rap Shoal, Hampton Roads, and Fort Delaware, 
Delaware River. 

18. Records of the Treasury Department. Reports to the Architect from the Con- 

structing Engineers on Foundations of Custom-house, New Orleans. 

19. Journal of the Franklin Institute for February and March, 1868. McAlpine's 

Iron Cylinder Foundation, Sunk by the Pneumatic System, modified by him, 
for the Bridge at Harlem, with Notes on Resistance of Iron and Wooden Piles. 

20. Report of the Light-House Board, accompanying the Report of the Secretary of 

the Treasury to Congress for December, 1868. Description of Pneumatic Pro- 
cess of Constructing Foundation for Waugoshance Light-house, Lake Michigan. 

21. Minutes of Proceedings of Institution of Civil Engineers, Vol. 23. Description 

of the Wrought-iron Light-house at Ushruffee, Red Sea. 

22. Engineer and Architectural Journal for January, 1868. Description of Wrought- 

iron Light-house for the Douvres, between the Islands of Guernsey and Brehat. 

23. Proceedings of Institution of Mechanical Engineers for 1861. Description of 

Wrought-iron Light-house at Buda, Spain. 

24. Encyclopaedia Americana. Pile Foundations of the City of Amsterdam. 

25. Engineer and Scientific Journal for 1867. On Pile-driving. 

26. Engineer and Architect's Journal. On Foundations for Susquehanna Bridge, 

April, 1867. For the Clyde Viaduct, November, 1864. For the Albert Bridge, 
Saltash, Cornwall, 1862 and 1864. On Bridge Pile Foundations, November, 
1864. Opinions on London Bridge Foundations, January, 1858. For Founda- 
tions on Compressible Soils and Enlargement of the Base, 1857 ; and on the 
Grimsby Dock Foundations, 1864. 



Treasury Department, 
Document No. 128 
Light-House Board, 



NT, } 

ON FOUNDATIONS IN COMPRESSIBLE SOILS. 



The following notes on the practice and experience of American and 
European engineers on foundations in very compressible soils are col- 
lected for the consideration of the committee of the Light-House Board, 
to which was referred the different projects heretofore submitted for a 
foundation for a light-house to be constructed at the Southwest Pass 
of the Mississippi river. 

The soundings off the mouth of the river to the westward along the 
Louisiana and Texas coasts, and to the northward along the Chande- 
leur Islands, and thence eastward along the coasts of Mississippi and 
Alabama, all indicate sandy bottom beyond the immediate influence 
of the rivers. The advance of the delta of the Mississippi into the 
Gulf of Mexico is composed of alumina and vegetable matter overly- 
ing this sandy bed. The depth of the sand below the waters of the 
river and gulf appears to be beyond our reach as a base on which to 
rest any artificial structure, and the surface of the soil created by the 
deposits of the freshets is so deficient in solidity as to be designated 
very appropriately " prairie tremhlante? We have to combat the diffi- 
culties presented by this overlying compressible mass. It is formed 
during the annual freshets of the rivers by deposits, in eddies and 
slack- water, of the matter abraded by the current from the shores and 
banks of the different rivers as their freshets are thrown into the 
Mississippi, and not thrown up or translated along the coast by the 
ocean wave. These deposits from the tributaries of the Mississippi 
valley vary in specific gravity, are deposited daring the annual freshets 
in proportion to their gravity, and, combined with more or less water 
in the porous mass, form strata of varying density overlying each 
other, and constitute the accumulation from year to year. 

A remarkable feature in this alluvial formation is the upheaval, by 
some unaccountable power, of islands that consist of the deep-seated 
strata, raised and forced up many feet above the level of the gulf and 
annual freshets, leaving cavities beneath them, and an element of 
destruction in the continued discharge of matter mixed with water 
thrown up for an indefinite period after their first and sudden eruption 



and formation. They have been sufficiently described in previous 
reports to the Board, on its files, and now only referred to for indicat- 
ing localities the engineer should studiously avoid as beyond his power 
of adaptation for foundations of heavy structures, and, with the pre- 
vious introductory remarks, to be considered with the projects of en- 
gineers for other difficult localities now to be noticed. (See House 
Doc. No. 7, 1st Sess. 21st Cong.) 

HOLLOW-IRON CYLINDERS SUNK BY ATMOSPHERIC PRESSURE. 

This system has been applied successfully in Europe and America. 
Having been proposed for the Southwest Pass light-house, the follow- 
ing notes are submitted explanatory of its advantages and applicability 
in certain localities: 

The first application of this principle is mentioned in Ure's Diction- 
ary of Arts and Manufactures as having been tried on the Loire, in 
France, by Triger, in sinking a shaft 65 feet. A detailed account of 
Mr. Triger's system is to be found in the Comptes Rendus de l'Acade- 
mie des Sciences. The principle was applied by Mr. Hughes in sinking 
the piles at Rochester bridge, England. With his modifications it has 
entirely superseded the ordinary diving-bell for foundations in deep 
water. Compressed air is made to free a hollow pile from the water 
within it after it has been placed in its situation, (the bed of the river 
or other place,) there used as a diving-bell, without again being drawn 
up, and remains as a part of the permanent structure. 

Mr. Hughes sunk, on the Rochester side of this bridge, 12 cylinders 
or piles, and 30 on the Strood abutment; each pile consisting of two or 
three or more sections of cylinders, 9 feet in length, 7 feet in diameter, 
bolted together through stout flanges, the bottom having a bevelled edge. 
For a description of sinking these piles, see Crecy's Supplement to his 
Encyclopaedia of Civil Engineering, London, 1856. Mr. Hughes's sys- 
tem is admirably shown on a large scale, with all its important details, 
in a work published by him in England. 

The agents of Dr. Potts, as a patentee, introduced it in this country, 
and it was successfully used by Mr. Gwynn for a railroad-bridge in 
South Carolina, in a sandy bed of the Peedee river. Mr. McAlpine has 
used it with success for a bridge over the Harlem river, N. Y., in a 
muddy and sandy soil, and at this time it is being applied on a more 
enlarged scale, under the Light-House Board, by General W. Sooy 
Smith, in the construction of a wall around the light-house at AVaugo- 
shance, in Lake Michigan, in a gravelly and rocky bed. 

The Clyde or Nethan viaduct is carried on cast-iron cylinders, sunk 



in the sandy bed of the river, filled up to the level of flood-tide with 
concrete, leaving upwards of .40 feet of the upper portion of the cylinders 
without any filling. (Engineer and Architectural Journal, November, 
1864.) 

In the construction of the centre pier of the Albert bridge, at Saltash, 
on the Cornwall railway, a wrought-iron cylinder, 37 feet in diameter 
and 90 feet high, open at top and bottom, was sunk through the mud to 
the rock. It was expected that forming a bank around the cylinder, 
after being sunk to the rock, would exclude the water. 

The cylinder was constructed to admit also of air-pressure; the sur- 
face of the rock was inclined 6 feet lower on one side than the other ; 
the iron cylinder was shaped to conform with the rock; a dome or lower 
deck was constructed inside at the level of the mud, 13 feet below the 
surface of the water; and an internal cylinder, open at top and bottom, 
connected the lower with the upper deck of the cylinder; a 6-foot cylin- 
der was fixed eccentrically inside the other, and an air-jacket or gallery, 
making an inner skin around the bottom edge below the dome, was 
formed about 4 feet wide, divided in 11 compartments, and connected 
with the bottom of the 6-foot cylinder by an air-passage below the 
dome. 

When the 37-foot cylinder was thus constructed, it was towed to and 
accurately adjusted over the intended site; water was then let in, until 
the cylinder penetrated through 13 feet of mud, and rested on some 
irregularity, causing it to keel over about 7' 6". By letting water in 
upon the dome or lower deck, and loading the higher side with iron 
ballast, the cylinder forced its way through the obstruction at the bot- 
tom edge, and took a nearly vertical position. 

The air and water-pumps were then worked, and the greater part of 
the mud and oyster-shells which filled the compartments of the air- 
jacket was cleared out, and the irregular surface of the rock excavated, 
the bottom of the cylinder being now 82 feet below high-water line. A 
ring of ashlar stone, 4 feet wide and 7 feet high, was then built in the 
air-jacket, and a bank of clay and sand was deposited around the out- 
side of the cylinder to compress the mud. 

When the water was pumped out the body of the cylinder below the 
dome, and the excavation of the mud was being proceeded with, a leak 
broke out, and the water overpowered the pumps. Recourse to air- 
pressure in the body of the cylinder below the dome was determined 
upon, and arrangements made therefor. 

The 37-foot cylinder was loaded with 750 tons of ballast, when the 
pumps succeeded in keeping the water down. The mud was then 



8 

excavated, the cylinder below the dome securely shored across, and 
the rock levelled ; when the masonry was commenced in the body of 
the cylinder. 

As soon as the masonry reached the level of the air-jacket ring, the 
plates of the air-jacket were cut out, and the two masses of masonry 
were bonded and thus united, forming a single mass. Upon the top 
of the bonding course of masonry two courses of brick were laid in 
cement, making a water-tight floor over the whole diameter of the 
column. 

The next operation was to draw off the water above the dome and 
remove the ballast, allowing the masonry within to proceed. After 
the masonry had been completed to the plinth course, the upper part 
of the cylinder was unbolted at the separate joints and floated to the 
shore. 

The roadway is 100 feet above high-water mark. This centre pier 
supports iron arches of 455' span each from the centre of the river. 
(Engineer and Architectural Journal, 1862 and 1864.) 

Bridge over the Harlem river, on cast-iron cylindrical piles, by Wm. 
J. McAlpine. The draw-pier of this bridge was composed of one central 
and ten circumscribing iron columns, each 6 feet in diameter and 50 feet 
in height — the water being 20 feet in the deepest part. These piles 
were sunk by the pneumatic process, both plenum and vacuum. It 
was deemed advisable to increase even the large base, due to the size 
of the column formed by these cylinders. 

It had been decided to fill the columns with concrete ; and it was 
suggested to extend this masonry below the bottom of the iron cylin- 
ders, (as the men could work in water,) undermining the adjacent earth 
as far as practicable, and to extend the concrete into the space thus 
undermined. This was done in sections of about 2 feet in width; and, 
when the rim had been completed, it was found that the column was 
virtually extended, and that the water would readily sink, under the 
pneumatic pressure, to a level with the bottom of the concrete, so that 
the sand within it was easily removed, and the space filled with con- 
crete to a depth, generally, of 4 feet or more. The cement set with 
greater rapidity under pneumatic pressure than in the open air. 

The last column was driven from 16 to 20 feet, in from 3 to 6 days,, 
in sand and porous material, free from obstruction ; 12 men, all told, 
sufficed to do the work, including engine-drivers, stevedores, and fore- 
men. The metal in the columns was 1 \ inches thick. No ill effects were 
experienced by the workmen from a pressure of 2J atmospheres. 

In some cases the pile could be sunk by the vacuum process alone j 



in all other cases by the plenum, and sometimes both might be em- 
ployed with advantage, and further aided by weight or pressure on the 
heads of the cylinders. 

The support of these iron columns, derived from friction on the ex- 
terior surface, was found to be half a ton per square foot, but in the finest 
earth it would amount to 3 tons. 

The support from the area of the bottom in shallow depths was from 
5 to 10 tons per square foot. (See Engineer and Architectural Journal 
for January, 1868.) 

This system is perfectly reliable in all cases where the compressible 
soil can be removed and a hard bottom reached on which to found the 
contemplated structure. It was practised with great success in the 
construction of the Theiss bridge, and minutely described in the An- 
nates des Ponts et Chaussees, 1859, and Annales du Ohimie, 1841. Mr. 
Mc Alpine gives much useful and practical information on this subject 
in the February and March numbers of the Journal of the Franklin In- 
stitute. For a foundation at Waugoshance, Lake Michigan, on this 
system, for a wall of 8 feet thick, and laid 1 2' 3" below the surface of the 
water, 7 feet of which was through gravel and large boulders, enclosing an 
elliptical area of 67' by 49', under one wrought-iron cylinder, with air- 
pumps and valves, see the Eeport of the Light-house Board, accom- 
panying the Eeport of the Secretary of the Treasury for December, 
1868, pp. 71 and 72. 

PILE FOUNDATIONS. 

The next system for consideration is that of piles driven into the 
ground, on which the superstructure rests directly, or through the in- 
tervention of a floor of wood or masonry. It is deserving of particular 
attention from its probable fitness for the locality under consideration 
at the Southwest Pass of the Mississippi river. 

FOUNDATIONS ON WOODEN PILES.— AMSTERDAM. 

The most extensive and oldest application of pile-driving for found- 
ations, to which instructive reference is at command, is in the city of 
Amsterdam. The original site of this city was a salt marsh. All the 
buildings (28,000) for a population of 224,000 souls (in 1850) covering a 
surface of 900 acres, are supported on piles of from 50 to 60 feet in length. 
After passing through a mixture of peat and sand of little consistence, 
at a depth of about 40 feet, they enter a bed of firm clay. The ends of 
the piles are sawed level and covered with thick plank, on which the 
masonry is constructed. Though the houses have declined from the per- 
pendicular, they are considered to be quite secure against falling; yet 



10 

such a contingency occurred in 1822, by the sinking and total ruin of 
a large stack of warehouses heavily filled with corn — (wheat in bulk 
that shifted the weight.) 

The palace, built in 1648, is supported on 13,659 piles. It is 282 feet 
long, 235 feet wide, and 116 feet in height, exclusive of a cupola of 41 
feet. 

The steeple of the Oude Kirk is 240 feet high. The surface or level 
of the natural ground is below the level of the ocean. 

FOUNDATIONS ON WOODEN PILES AND CAISSONS IN THE BED OF THE 

THAMES, LONDON. 

The labors of engineers in the valley of the Thames river, at and 
about London, give much useful information in the construction of 
ancient as well as modern pile foundations. 

Old London bridge was commenced in 1176, and finished about 83 
years thereafter. The piers rested on piles driven only around the out- 
side of the pier, so placed as to carry the whole weight They were of 
elm, and at the expiration of six hundred years, on being drawn up, 
remained without material decay. A part of this bridge fell about 100 
years after it was finished; and the whole structure was removed to 
give place to the new bridge in 1825. 

Old Westminster bridge was built between 1733 and 1747. Piles were 
used under one pier only. Its piers (with the one exception) were con- 
structed in caissons or flat-bottomed boats. Each caisson contained as 
much timber as a forty-gun frigate. They were 80 feet long by 30 feet 
wide. These foundations failed; the caisson bottoms or floors sunk in 
the middle; the sides and ends projecting beyond the stone-work broke 
off and bent upwards. The sinking of the bearing area of the sub- 
stratum, under the partial and unequal pressures of the floors of the 
caissons, has had more to do with the failure than any other defect. 

Black Friar's bridge was built between 1760 and 1771. The founda- 
tions were laid in caissons, the floors of which rested on piles about 9 
feet apart — only 45 piles to each caisson — and intended to obtain a level 
surface on which to rest the floor, to settle down to a uniform bearing. 
Its stability, says Eennie, for many years is to be attributed to these 
piles. It has since failed, and requires large expenditures on repairs. 

It was believed in this case, as at Westminster, that if the scour of 
the river bed could be prevented, and nothing carried away from under 
the foundations and about the piles by external agency, the clay base 
would support the gravel stratum above it, and the gravel the stone ; 
but the pressure per foot in the Westminster was nearly 5J tons, and 



11 

in Black Friar's 5 tons per square foot 5 and clay, when the pressure is 
at great depths within it, will not bear 5 tons per foot. 

Waterloo bridge, built between 1809 and 1817. The foundations of 
the piers of this bridge are built in coffer dams or caissons, the floors of 
which rest on piles, about 3 feet apart, under the entire base of the pier, 
penetrating the clay about 18 feet. The foundation is arranged with 
plank, concrete, and stone, as in the New London bridge. The esti- 
mated pressure per foot on the head of each pile is, in this case, about 
68 tons. The arches are 120 feet span, and weigh about 2,500 tons each. 

Vauxhall bridge was built between 1811 and 1816. The foundations 
of the piers of this bridge were built in caissons. An excavation was 
made down through the gravel stratum to the clay. It is a light struct- 
ure, and no settlement is recorded. 

New London bridge, built between 1825 and 1831. The foundations 
of the piers of this bridge are built in coffer dams or caissons, the floors 
of which rest on piles under the entire area of the bases of the piers. 
The piles are about 20 feet long and 3 feet apart, penetrating the clay 
18 to 19 feet. On the heads of the piles were laid sleepers ; the loose 
earth between the heads of the piles is replaced with rubble concrete, 
on which blocks of stone and brick- work filled up the spaces between 
the pile-heads and immediately over the platform of oak planking which 
carried the first course of granite. 

The pressure upon each pile is 80 tons, or 5 tons per square foot of 
the entire area of the pier. Each pier of this bridge settled from six 
to ten inches towards the down stream. No further settling is appre- 
hended. 

Hungerford bridge, built in 1844. This is a chain suspension bridge. 
On the Hungerford-market side the ground under the mooring piers 
was very bad. Piles were here driven to the depth of 30 feet. 

Chelsea bridge, built between 1850 and 1857. This is a suspension 
bridge. The foundations of the piers are on bearing piles of 14 inches 
square, 3' 6" apart, and driven 32 feet below low water, and about 16 
feet into the London clay. For other details of these foundations, see 
Engineer Architectural Journal, for November, 1864. 

New Westminster bridge, built in 1858 and since. The arches of this 
bridge are of wrought and cast iron. Elm piles, 32 feet long, are driven 
in alternate rows of 3 and 5 each, to the number of 145 in each pier, 
and 18 to 20 feet into the London clay, each tested to a bearing weight 
of 60 tons. Circumscribing the area into which the elm jules are driven, 
hollow cast-iron piles, 15 inches diameter, 25 feet long, are driven 4 feet 
apart, and between these, in grooves cast on hollow piles, flat iron piles 



12 

are driven to nearly the same depth, forming a coffer dam, within which 
all the soil overlying the gravel bed is excavated, and concrete filled 
in to the top of the piles, which are cut off 6 inches below low water. 

On the heads of the elm piles is a course of stone covering two or 
three piles alternately ; upon this the bottom course of granite of large 
size is laid. The bearing piles are 14 inches square, driven at intervals 
of 1' 9" from centre to centre, to an average depth of 20 feet in the 
London clay. It will thus be noticed that the solid stone piers sustaining 
the iron arches rest upon the heads of the wooden piles, which stand 
a considerable height above the bed of the river ; that these piles are 
within an enclosure of cast iron and granite slabs ; and around the piles, 
and filling up the sides of the casing or enclosure, there is a solid bed of 
concrete as good as rock. 

At the Hull docks the load per pile per square foot is 37 tons ; at the 
London bridge the load per pile per square foot is 80 tons; at the Albert 
warehouse, Liverpool, the load per pile per square foot is 80 tons j and 
at the New Westminster bridge it is 12 tons per square foot. The 
pressure on the whole area of the foundation is only 2 tons, while on 
the old bridge it was 6 tons, and in the London bridge it is 5J tons. 

REMARKS OF ENGLISH ENGINEERS ON THE LONDON BRIDGES. 

The bed of the Thames is much lower than when the bridges which 
have failed were built, occasioned by the increased scour produced by 
removing the old London bridge. Whenever the foundations have 
been made to depend on the gravel, and have not been taken deep into 
the London clay, failure has taken place. In the clay only can a founda- 
tion be found. 

The old successful examples are all of one class — coffer-dam examples, 
excepting Yauxhall and Mr. Page's recent structures. Vauxhall was a 
coffer-dam carried to the clay without piling; the weight of the bridge 
not needing piles. 

All the other sound bridges are piled deep into the blue clay. The 
shoulders and sides of these piles, and the surface of the clay between 
them at their tops, are the ultimate bearing points upon which presses 
the superstructure, whether of granite or other stone courses, or com- 
posed of concrete and wood, or of iron. 

The concrete and iron casing about the piers of the Westminster, is 
believed to be a ten times stronger medium, for the retention of the 
piles in their places, than the London clay (at whatever depth) would 
be; and omitting, therefore, all aid from the external piles and casing 
in bearing the weight, we find that the 133 elm bearing-piles are alone 



13 

capable of bearing a load four times greater than can be put upon 
them, and there is not so much chance for unequal settling as with sleepers 
and bearing-planks, such as exist at the successful bridges. We cannot 
find one instance of deep-piled bridges abroad which has fallen. 

In Venice the piles are covered with a planking of fiat boards under 
water. Whilst in London clay would only bear a pressure of 5 tons 
per foot, piles driven into it would carry 70 to 80 tons. 

At the Hull docks the piles were 10 inches in diameter, and carried a 
weight of 37 tons per foot superficial: at the London bridge the piles 
were 10 inches in diameter, and carried a weight of 80 tons per foot ; at 
the Albert docks, Liverpool, the piles sustained a weight of 80 tons per 
foot ; at the New Westminster bridge the piles were 14 inches in diam- 
eter, and would have to carry only 12 tons per foot. It was stated that 
the elm piles used in the foundations of Westminster bridge would 
carry 200 tons without permanent deflection. (See Engineer and Ar- 
chitectural Journal, for 1858.) 

The compressibility of oolitic and tertiary clays can only be overcome 
by piling, deep sinking, heavy ramming, or heavy weighting . The point 
of bearing must be carried below the possibility of upward reaction. 
The depth of a foundation in compressible ground ought not to be less 
than one-fourth the intended height of the building above ground — 
that is, for a shaft of 200 feet the foundations should be made secure to 
a depth of 50 feet, by piling or by well-sinking and concrete. (Engi- 
neer and Architectural Journal, for 1857.) Masses of concrete, brick 
or stone, placed on a compressible substratum, however cramped and 
bound, may prove unsafe. Solidity from a considerable depth can alone 
be relied upon. Mere enlargement of a base may not in itself be suffi- 
cient. (Engineer and Architectural Journal, for 1857.) 

FOUNDATIONS OF THE GRIMSBY DOCKS ON THE HUMBER. 

These docks were commenced in 1846. The entrance to them is be- 
yond the low-water line, and advanced into the river three-quarters of 
a mile. The ground over the whole area of the two entrance locks, 
centre pier, and wing walls was excavated 8 feet below the sill of the 
larger lock, and bearing-piles were driven in rows 5 feet from centres, 
and in some places 4 feet over the whole area. A pile was considered 
sufficiently driven when it moved not more than one- quarter of an inch 
with the blow of a ram of 1 ton falling 12 feet. The heads of the piles were 
then cut off to a uniform level, the ground was removed to a depth of '2 feet 
below this level, and the spacefilled with concrete. Timbers so connected 
as to form continuous ties across the locks and centre pier were then 



14 

laid transversely, in parallel rows, on the bearing-piles. Other similar 
timbers were laid at right angles to the transverse bearers, concrete 
being filled in to the upper surface of these longitudinal bearers, which 
were then covered with planking as a bed for the masonry. (Engineer 
and Architectural Journal, for 1864.) 

The result of European experience gives the following as the bearing- 
weights supported by the foundations of the piers of several of the 
most remarkable and heaviest structures, in pounds, per square foot: 

Dome of St. Peter's, Eome 35, 254 lbs. 

Dome of St. Paul's, London 41, 713 lbs. 

Dome of the Invalid, Paris- 31, 862 lbs. 

Dome of the Pantheon, Paris 43, 440 lbs. 

Column of the Basilica of St. Paul 42, 950 lbs. 

Steeple of the Church of St. Mary 63, 325 lbs. 

For the relation between the total surface covered and the part occu- 
pied by its walls , or the supporting parts and the above, see Rondelet, vol. 
3, p. 232. 

PRACTICE AND EXPERIENCE OF UNITED STATES ENGINEERS. 

We now proceed to give the practice and experience of the engineers 
of the United States, in the construction of foundations in different 
localities and in compressible soils. The dry-dock at the Brooklyn 
navy-yard, New York, was commenced in 1841 and completed in 1851. 
It contains 13,837 cubic yards of masonry, resting upon 38,532 cubic 
feet of pile timber. 

The soil was found to be chiefly vegetable decomposition to the depth 
of 10 feet, and below this almost impalpable quicksand containing a 
large proportion of mica. When confined, and not mixed with water, it 
is very firm and unyielding, presenting a strong resistance to penetra- 
tion. When saturated with water it becomes a semi-fluid, and is moved 
by the slightest current of water passing over or through it. Small 
veins of coarse sand were also occasionally encountered, through which 
flowed springs of fresh- water. Borings were made to the depth of 80 
feet, and brought up sand and clay and fresh- water. There is but a 
small proportion of clay in any part of the foundation. The borings 
extended 40 feet below the foundation of the dock. 

The foundations for the superstructure of this dry-dock were placed 
37 feet below mean tide and 42 feet below the surface of the ground. 
Black mica overlaid the quicksand under the coffer-dam. Under and 
in this coffer-dam 3,504 piles were driven, averaging 39 feet in length 
by 15 inches square. 



15 

The earth above low water was removed before the coffer-dam was 
formed, and about 10 feet in depth was removed by dredging. The 
semi-fluid state in which the material was found after the water had 
been pumped out of the pit was very difficult to remove. It was so 
fluid as to require tubs for its removal. Bottom springs of fresh-water 
were found in about 6 feet of the required depth of the foundations. 
The largest discharged 10 gallons per minute. When flowing from a 
level of 26 feet below low water, it discharged 38 gallons per minute, 
containing 27 ounces of sand ; at a level of 22 feet, it discharged 33 
gallons per minute, containing 17 ounces of sand ; at a level of 17 feet, 
it discharged 10 gallons per minute, unmixed with sand. 

These springs presented great difficulties in laying the foundations 
from the flowing of the water, which as it came up brought large 
quantities of sand, which, if continued to flow, would soon have en- 
dangered the surrounding works. The pressure of the water was so 
great as to raise the foundation, however heavily it could be loaded. 

The settling of the piles supporting the pump-well, was the first 
evidence of undermining from one of these springs. The site of the 
well was changed, but the spring followed, and compelled another 
change of the well. 

This spring was driven out of the old well by driving piles until it was 
filled up,but it immediately burst up among thefoundation-piles of the dock 
near by. In a single day it made a cavity in which a pole was run do wn 20 
feet below the foundation- timbers. One hundred and fifty cubic feet of 
stone were thrown into this hole, which settled 10 feet during the night, and 
50 cubic feet more were thrown in the following day, which drove the 
spring to another place, where it undermined and burst up through a 
bed of concrete 2 feet thick. This new cavity was repeatedly filled 
with concrete, leaving a tube for the water to flow through ; but in a 
few days it burst up through a heavy body of concrete in a place 14 
feet distant, where it soon undermined the concrete, and even the 
foundation piles, which settled from 1 to 8 inches, although 33 feet 
long, and driven by a hammer of 2,200 lbs. falling 35 feet at the last 
blow, with an average of 76 blows to each pile, the last blow not moving 
the pile half an inch. It was then determined to drive as many additional 
piles into the space by means of followers to force those already driven 
as deep as possible. 

The old concrete was then removed to a depth of 20 inches below 
the top of the piles. An area of about 1,000 square feet around the 
spring was then planked, on which a floor of brick was laid in dry 
cement, and on that another layer of brick set in mortar. The space 



16 

was next filled with concrete, and the foundations completed over all. 
Several vent-holes were left through the floor and foundations. After 
a few days, when the cement had well set, the spring was forced up to 
a level of about 10 feet above the former outlet, at which it flowed 
clear without sand. 

Two other of these springs were closed by freezing in 1848, and forced 
up in one case 800, and in the other 1,200 square feet of the founda- 
tions. This took place between the lower timbers and the planking, 
lifting also the first course of the stone floor, which was from 12 to 15 
inches thick. 

The whole number of bearing-piles in the foundation is 6,549. They 
are chiefly round spruce timber 25 to 40 feet long, averaging 14 inches 
diameter at the head. The average length of all the piles driven was 
32/ 7//. The piles were originally driven 3 feet from centres. After- 
wards as many piles were driven as could be forced into the earth. 
Whenever a hammer of 2,000 lbs. weight, falling 35 feet, drove the pile 
for the last few blows exceeding 3 inches per blow, another and larger 
pile was driven alongside. 

With the exception of 541, all these piles were driven by hammers 
from 2,000 to 4,500 lbs. each, falling from 35 to 40 feet. The average 
number of blows per pile was 151 with the small hammers, and 50 blows 
only per pile with the large hammers. The 541 piles were driven with 
a Nasmyth steam-pile engine. 

A trial round pile of 20 inches diameter at the butt and 14 inches at 
the small end, of 49 feet in length, was driven by a 2,024-lb. hammer, 
falling finally 35 feet — 45 feet below the foundations. The first 100 
blows the hammer fell but a few inches ; the next 260 blows drove 
the pile 30 inches in 46 minutes ; the next 260 blows drove \ to 1 J inches 
per blow for 60 minutes ; the next 110 blows averaged \\ inches per 
blow for 60 minutes, the hammer falling the last blow 34 feet. 

This trial-pile subsequently received 200 blows through the medium 
of a follower, which drove it an average of half an inch to each blow. 

Another trial-pile was driven 43 feet by aNasmyth steam- pile driver, 
and then another pile, 15 feet long, driven on top of the first ; making 
a total penetration in the earth of 57 feet. 

The first pile was driven 42 feet by 373 blows in 7 minutes, as fol- 
lows: 

4 blows, 4 inches each. 






8 blows, 3J inches each. 
22 blows, 3 inches each. 
25 blows, 2 inches each. 



L7 

40 blows, 1J inches each. 
56 blows, 1J inches each. 
32 blows, 1J inches each. 
64 blows, 1J- inches each. 
73 blows, 1 inch each. 
49 blows, i inch each. 

The second pile was driven 15 feet by 2,400 blows in 43 minutes, as 
follows : 

33 blows, f of an inch each blow. 

73 blows, J of an inch each blow. 
100 blows, J of an inch each blow. 
800 blows drove it altogether 88 inches. 
300 blows drove it altogether 24 inches. 
300 blows drove it altogether 12 inches. 
450 blows drove it altogether 11 inches, (and the last.) 
350 blows drove it altogether 5 J inches. 

The movement of these piles indicated the continuance of the same 
material to the depth which they reached. 

The foundation was mostly laid as follows : The excavation being 
completed to the proper depth, and piles cut off to a uniform level, 2 
feet in thickness of concrete was rammed between the bearing -piles. These 
piles were then capped with 12 and 14 inch yellow-pine timber, laid 
transversely with the axis of the dock, and treenailed to each pile. The 
concrete was then raised to the top of these timbers, and a light floor- 
ing of 3-inch yellow-pine plank was laid upon and spiked thereon. 
Another course of similar timber was then placed upon this floor, 
breaking joints with those below, to which they were treenailed. The 
intervals were next filled with concrete, and another floor of 3-inch 
plank spiked down, which completed the foundation. 

The amount of work done by the heavy Nasmyth hammers was at 
least one-third greater than that done by those whose hammers were 
only one-half the weight. This Nasmyth hammer of 4,500 lbs. was 
worked with very short, rapid blows, raised a height equal only to the 
stroke of the engine. 

The support of this foundation is derived mainly from the adhesion 
of the 'material into which the piles were driven, and slightly from 
their sectional area. 

It was ascertained that it required a weight of 125 tons to draw up 
one of these piles, or rather to start or put it in motion, when driven 
33 feet to the point of ultimate resistance, with a ram of 1 ton falling 
2 M 



18 

30 feet at the last blow. The piles averaged 12 inches in diameter 
in the middle, making at least a support of 100 tons per square foot of 
foundation. 

PILE FOUNDATIONS AT THE PHILADELPHIA NAVY-YARD. 

At the site of the dock of the Philadelphia navy-yard, commenced 
in 1849, the first stratum of soil is a mud of rich loam, extending from 
a little above low- water level, declining towards the bottom of the 
river, which is of a clean gravel, to the depth of about 24 feet below 
ordinary low tide, and evidently the deposit, through a long period of 
years, of the earthy matter held in suspension by the waters of the Del- 
aware river, upon a stratum of sand and gravel forming the bed of the 
river. The sand and gravel are not more than from 4 to 7 feet in' 
thickness before those substances become mingled with paving-stones, 
large boulders, and to some extent with clay, forming a species of hard 
pan, to which all the piles used in the construction of the foundations 
of the work were driven. The object of the foundation of the basin of 
the dock, was to give support to its bottom, and to secure its outer 
edge against the action of the current of the river. As the excavation 
was completed, piles were driven 4 feet apart from centres in rows from 
one end of the space to the other. An extra row was driven under the 
line of the walls of the three sides of the basin. A space was then 
formed by drawing a line of sheet-piling 8 feet from the inner line of 
piles of the coffer-dam. Two extra rows of piles were driven within 
this space, which was then filled with concrete to within 2 feet of the 
floor of the basin. The piles thus driven were cut off to the same level, 
capped with timbers 1 foot square, and the spaces between these cap- 
ping-timbers filled with earth and concrete. (Stewart's Dry Docks of 
the United States.) 

PILE FOUNDATIONS AT THE PENSACOLA NAVY-YARD. 

The dock at the Pensacola navy-yard was built in 1851 and 1852. 
The soil of this locality is clean white sand to a depth of about 40 feet, 
resting upon a bed of soft clay. The sand is so open and porous that 
a cubic foot of it, when saturated, contains six quarts of water. 

A space of 140 feet wide by 380 feet long was enclosed by driving 
yellow-pine piles, 12 inches square, to the depth of about 20 feet into 
the sand, placed in contact with each other, for forming a coffer-dam. 
"Within this space, secured by other means against filtration, the sand 
was excavated to a depth of 14 feet below tide. 

After a section of the pit had been excavated to the level of the 






19 

foundations, the bearing-piles were driven in rows 4 feet apart and 4 
feet from centres in each row, until a ram of 2,200 pounds, falling 30 
feet, could not move them more than half an inch. Upon the transverse 
rows of piles cap timbers, 12 inches square, were placed, and the space 
between the timbers filled with sand. The timbers were then covered 
with 5-inch plank, spiked to the caps, and the whole floor calked with 
wedges. On this floor the masonry of the basin was commenced. This 
foundation was tested and found satisfactory. The experimental tests 
showed that a single foundation-pile, as a, fulcrum, sustained nearly 39 
tons without settlement ; and it required 41 tons strain to draw a pile 
that had been driven 16 feet into the sand. 

The railroad-bridge at Ha vre-de- Grace, on the Susquehanna river, 
was commenced in October, 1862, and finished in November, 1866. 

The foundations of the piers of this bridge are referred to as ex- 
amples of piles driven in the compressible bed of the river, supporting 
an iron caisson on a grillage of timber resting on the piles, and a cais- 
son lowered to solid rock through 15 feet of compressible soil. 

In 1863, piles were driven and sawed off 40 feet below the surface of 
the water for the foundation of pier No. 3 ; a platform or grillage of 
timber, strongly ironed, upon which the pier was to rest, was con- 
structed near the site of the work and floated over the site of the foun- 
dation under which the piles had been driven. This platform was placed 
between two substantial construction piers of timber ; lowering screws, 
6 in number, of 3£ inches diameter, were attached by hooks to the plat- 
form; and to the construction piers a section of an iron caisson was 
constructed, resting upon the wooden platform thus suspended. The 
masonry was then built within the caisson, lowered by means of the 
screws as it approached the top of the section, when a second section 
of the iron caisson was added, built within and lowered by the screws 
in like manner as the preceding, and thus continued until the grillage 
or wooden platform rested upon the heads of the piles. 

Another pier (No. 7) was founded on the rocky bed of the river un- 
derlying the compressible bed of the stream for a depth of 15 feet. This 
latter had been displaced in several places by the spring freshets of 
1865, rendering piling impracticable ; all the earth was, in consequence, 
removed down to the surface of the rock. At the site of this pier the 
rock bed was 18 feet below the original undisturbed bed of the river. 
To remove this earth, a wrought-iron foundation caisson, averaging 8 
feet in height and about 50 by 20 feet square, was lowered so as to 
enclose the site of the pier. This was gradually depressed to the rock 
by removing the earth within it by means mainly of powerful pumps, 



20 

aided by the constant exertions of skilled divers. The masonry was 
then laid within the tank upon the solid rock, and being brought to a 
level some feet below the top of the foundation caisson, the caisson of 
the pier resting upon the platform of timber was lowered and built 
upon. The foundation of this pier was 36 feet below low water. (See 
Engineer and Architectural Journal, April, 1867.) 

RESISTANCE OF PILES TO PRESSURE.— FORT RICHMOND, ST ATEN ISLAND, 

NEW YORK. 

The soil of part of the foundations of Fort Eichmond, now Fort Wads- 
worth, on being excavated to low- water level, was found to be very 
compressible, with springs discharging large quantities of water. The 
scarps and counterscarp of the southern half of the land front and face 
of the adjacent bastion of the water front were situated on such a soil, 
resembling that of the dry-dock at Brooklyn. The weight to be sus- 
tained was a granite casern ated battery of four tiers of 8 and 10-inch 
guns, with the shot, shell, and other munitions therefor. The scarps,, 
piers, and arches are all of granite. 

Piles were in this case resorted to, and no settlement has since been 
noticed. The work was commenced in 1856, and finished to receive its 
armament in 1861. The piles were 30 feet long, 12 inches square at the 
head, and not less than 10 inches at the small end. They were driven 
by a hammer of 1,800 pounds weight, with blows in quick succession r 
the last blow of the hammer being from a height of 45 feet. They 
were cut off level with the surface of the ground, and capped with 
large flat stones, covering the heads of from 3 to 5 piles — the joints be- 
ing filled with concrete, well rammed. Upon this was laid a second 
layer of large flat stones, breaking joints with the first, and on this the 
masonry of the granite scarp was commenced. It will be noticed that 
there was no timber grillage covering the heads of these piles. 

RESISTANCE OF PILES TO PRESSURE.— EXPERIMENTAL TESTS AT 

PENS AC OLA. 

In 1851, a series of experiments were made, under the direction of a 
special board of officers, on the resistance, &c, of the piles driven for 
the foundations of the dock of the navy-yard at Pensacola. A pile 
that had been driven six days, surrounded by other piles 4 feet dis- 
tant from centres, of 30 feet in length, averaging 13 inches diameter 
about the middle of its length, round, with bark on, was first selected 
for these experiments. The average depth of the pile in compact sand 
was 15 feet, driven by a hammer of 4,087 pounds and 69 blows, at the 
rate of 2J blows per minute. 






21 

The first experiment, made on the 17th of May, was the application 
of a power (by pressure or loading) of 23,850 pounds to a pile for 5 
minutes, which it bore without moving. The second experiment was 
by applying a power of 20,000 pounds to another pile; a third pile was 
subjected to the same strain for 5 minutes, and a fourth pile resisted 
the same power for 5 minutes. There was then applied to this last 
pile 31,360 pounds (14 tons) for several minutes. Three other piles 
were subsequently tried, and each resisted 22,400 pounds (10 tons) 
pressure for 5 minutes. 

The next test was by pulling up the piles, beginning with 22,400 
pounds, applied to the pile No. 4, before noted, increasing the power 
1 ton every two minutes — that is, after 10 tons had been applied for 2 
minutes; then 11 tons were applied for 2 minutes; then 12 tons, and 
so, until 15 tons, or 33,600 pounds pressure, had been tried, when a 
hook of the steelyards broke. Three days afterwards the experiment 
was resumed, beginning with 10 tons, gradually adding ton by ton, 
until there were 31 J tons of upward strain acting upon the pile. This 
experiment lasted 1 hour and 40 minutes, during which time the power 
upon the pile was constant. 

There was afterwards a gradual increase of power to 39 tons without 
moving the pile. It then resisted 40 tons for half a minute, when 
it began to rise slowly. Forty-one tons were then applied for lg minutes ; 
then 41J tons for half a minute, the pile meanwhile moving upwards at 
the rate of -^ of an inch per minute. The tests of 40, 41, and 41J tons, 
occupied half an hour, during which, the strain being constant, the pile 
was moved 2J inches. 

It was then subjected to a strain of 30 tons for 18 hours, and was not 
moved by it; after which, by adding from 2 to 7 tons, the pile was 
moved, in one hour, three (3) inches; and, after it had moved upwards 
about 6 inches, a power of 25 tons was allowed to remain on it for 2 days 
and did not move it. The pile was then withdrawn from the sand. Its 
extreme length was 29 feet. The length of the part which had been 
in sand was 16 feet, including the sharpened end of 2 feet in length; 1 
foot in depth about this pile was loose sand, which had been once ex- 
cavated and fallen back. The average diameter of the part in the 
sand was 13J inches, including the bark. The bark remained on the 
entire pile, except on 3 J feet of its pointed end. This pile weighed 
1,632 pounds. 

A single pile, used as a fulcrum for the lever, sustained at the most, 
during the experiment, 38 T 9 \ tons weight without settlement. Ex- 
periments showed that piles which one day penetrated ^ of an inch 



22 

per blow, of a 4,087-pound hammer, falling 10 feet, was found to pene- 
trate J, J, and -j% of an inch, by three similar and successive blows applied 
the following day — the three blows being given in 1 minute. If the 
pile is allowed to remain undisturbed a short time, the power to move 
it afterwards is greatly increased. (Stewart's Dry Docks of the 
United States.) 

McALPINE AND SANDERS' FORMULA FOR BEARING WEIGHT OF PILES. 

The formula deduced by the engineer, Mr. McAipine, from his labors 
at the Brooklyn navy-yard, applicable to rams of 1,000 to 3,000 lbs. r 
falling from 20 to 30 feet, was ^ = 80 (W + .0228 VF— 1,) in which 
X was the supporting power of a pile driven by the ram W, falling a 
distance F; % and W being in tons, and F in feet — and it being under- 
stood that not more than one-third of the result given by this ex- 
perience should be borne or relied upon for any piles. 

Major Sanders' formula, deduced from his experience in driving piles 
at Fort Delaware, with a ram of 3,500 lbs., falling 3 J feet, driving a 

R h d 
pile 4'.2 ; is 3500X g 2 + 4V2 = ^2 = 4,375 lbs., the weight which the pile 

would bear with safety. (See Stewart's Dry Docks of the United 
States, and Engineer and Scientific Journal, for 1867.) 

FOUNDATIONS OF FORT MONTGOMERY ON PILES AND GRILLAGE. 

The foundations of Fort Montgomery, on Lake Champlain, near the 
latitude of 45°, were commenced in 1844, and the driving of 4,383 piles 
was finished in 1846. These piles were capped with a timber grillage 
in 1848. Each pile was calculated to sustain a weight of 7 cubic yards 
of masonry and If yards of earth, or a total weight of 34,125 lbs., or 
15.27 tons. 

The piles were driven about 3 feet apart. The grillage is in two 
courses ; the lower one, 1 foot 3 inches wide and 12 inches thick, is notched 
down 4 inches on the heads of the piles and pinned thereto. The upper 
course, at right angles to the lower one, is 12 inches square across the 
piles, and 12 inches by 8 inches between them, the 12 inches by 12 
inches being notched 4 inches down on the lower course, thus giving a 
level floor for the masonry. The length of the piles, after being driven 
and cut off, was from 29 to 33 feet. The fall of the hammer at the last 
blow was from 35 feet 8 inches to 36 feet. The diameter of the piles 
was from 12 to 16 inches at the butt, and 9J to 14 inches at the smaller 
end. The last penetrations were from 2 J to 6 J inches. These facts 
apply to 39 piles given as examples and illustrations of the whole. 



23 

The hammer weighed 1,630 lbs. The weight of the spruce piles was 
about 39 J lbs. to the cubic foot. The pile weighed was rather more dry 
than the average. At least 40 lbs. to the cubic foot would be a proper 
average. 

The compressibility of the soil into which these piles were driven was 
about one-ninth (-J-) of its entire bulk. 

COLONEL MASON'S FORMULA FOR BEARING WEIGHT OF PILES. 

Colonel Mason deduces the following formula from an investigation 
of the value of this foundation, or the amount of weight with which 
each pile may be loaded: Let h be the height of the fall of the hammer 
at the last blow, s the distance to which the pile penetrates at the last 
blow, 1,630 lbs. the weight of the hammer, 960 lbs. the weight of a pile 
24 cubic feet, and joint weights of pile and hammer 2,590 lbs.; g the 
force of gravity (32^ feet), or the velocity that gravity will generate in 
a second of time, and V 2 g li = velocity of hammer as it strikes the 
pile; - is J^_ = 115.2, and we have for the value of the retarding force 

1,630 x m% X 115.2 = 118,175 lbs., or 52.75 tons. Each pile was 
actually loaded with and supported the average weight of 28,575 lbs., 
or 12.75 tons, besides its own weight (960 lbs.) and that of the grillage, 
or about one-fourth of that which the calculated force of resistance is 
capable of holding in equilibrio. 

In the above case, the velocity of the hammer at the moment of impact 
will be V 2- x 36 x 32£ = 48.125 feet per second, and the velocity of the 

h g 

joint mass of hammer and pile will be 48.125 x yffo = 30.3 feet per 
second = V, and the retarding force ~ 2 will be ?£? = £J? = 1,468, 

it 8 7 .o 7 .o 

the velocity that the retarding force is capable of destroying m a second 
of time on the joint mass of the pile and hammer is then 1,468 feet. 
But gravity is capable of generating on that mass a velocity of 32-J- 
feet ; dividing 1,468 by 32|, we have 45.63 as the ratio between the 
retarding force and the force of gravity. That is, the retarding force is 
capable of holding in equilibrio 45.63 times the joint weight of the pile 
and hammer , or 2,590 pounds x 45.63 = 118,181 pounds. This is within 
six (6) pounds of the previous result. The time (t) in this case will 

be t = ~ = skt^o = 57 of a second. 

The same result may be obtained by dividing the velocity destroyed, 
viz., 30'.3, by the velocity the force is capable of destroying in a second 

30 3 2 

of time, viz., 1,468 ; or j-^ = ^, on the supposition of a constant force 



24 

during the short period that it takes to destroy the motion of the pile, 
instead of a retarding force. Colonel Mason concludes his memoir with 
the deduction that Fort Montgomery experience is in favor of a co-effi- 
cient of stability of 3 t 6 q when applied to the calculation under the sup- 
position of a constant force, although it in reality, as a general rule, 
decreases by very small decrements towards the terminal penetration; 
and hence, taking this co-efficient as a constant, is erring on the safe 
side. For a more minute detail of Colonel Mason's investigation of 
this subject; see papers on Practical Engineering, No. 5, by Colonel J. 
L. Mason, Corps of Engineers, TJ. S. Army, 1850. 

Major Sanders' experiments at Fort Delaware were made with a view 
of deducing a rule for foundation on compressible soils, without any 
firm substratum lying within reach of piles, for calculating what weight 
each pile would bear with safety, by comparing the distance it was 
sunk at the last blow with the force of the blow, it being understood 
that the pile has been driven to such an extent that for a number of 
blows the penetration has been uniform for equal blows. To ascertain 
such a rule two sets of piles of four each were driven, and a platform 
built on their heads ; they were then loaded with blocks of stone piled 
regularly on the platform, and at regular intervals of time the amount 
of subsidence caused by the weight, which was periodically augmented, 
was noticed. These piles were not driven through the alluvial ; their 
points were about 20 feet from the sandy subsoil, so that their stability 
was due to the accumulated and constantly increasing resistance of 
the same medium. 

In one experiment the four piles were driven to a depth of about 24 
feet each, with a pile-driver that struck 34 blows in a minute, with a 
ram of 2,000 pounds, and a uniform fall of 6 feet. An artesian well, 
sunk on this island in 1834, found mud continuously to a depth of 46 
feet below low-water mark, then 20 feet of sand, then 30 feet of coarse 
sand containing shells. It then entered and penetrated for 47 feet 
a bed of marl, which contained boulders. At the penetration of 
24 feet, each of the blows of the ram drove them about an inch, or ex- 
actly 7V of the fall of the ram. The weight placed on them at first 
was about 60,700 pounds, or about one-ninth of the product of the 
friction and the weight of the ram. This weight caused a subsidence 
in six months of / 2 °f an inch, when 15,000 pounds were added with- 
out increasing the subsidence. Upon again increasing the weight till 
it reached 94,000 pounds, (the ratio corresponding to which was one- 
sixth,) the subsidence began again, and in a month had attained ^ of 
an inch, when it ceased, or at least made no progress during the ensuing 



25 

three months. The next addition to the weight brought it to 107,500 
pounds, and the ratio to about one-fifth, and in a month increased the sub 
sidence to ■£% of an inch. The next addition raised the weight to 
121,800 pounds, and the ratio to 1:4.6. This weight remained upon the N 
piles without alteration till three years and five months had elapsed. 
In the first eight months it had caused the subsidence to increase from 
T2 t° -32" 0I> an i RCn 5 an( * this subsidence did not increase during the 
remaining two years and nine months of the period. During the en- 
suing seven months the weight was successively increased to 134,000, 
147,000, 160,000, 174,000, and finally to 189,500 pounds, (or 84.59 tons;) 
the ratios corresponding to which were respectively 1:4.19, 1:3.81, 
1:3.52, 1:3.23, and 1:2.969. Until the last weight was put on no addi- 
tional subsidence was caused by the increased pressure. It will be 
seen that the weight, answering to the ratio of 1:4.6 above mentioned, 
remained upheld three years and four months without the slightest 
variation in the subsidence of the piles, although it was repeatedly 
added to, and the whole system jarred in doing so during that period. 
But upon laying of the last load of stone the subsidence again began, 
and at the end of the ensuing period of one year and five months had 
arrived at ff of an inch. This subsidence did not go on uniformily, 
but by steps at a time. It seemed to be ceasing at the end of the 
period, judging from the facts that the intervals of rest were longer 
and the successive subsidence less towards that date, viz: April 12, 
1856. The next time that the observations were made on the levels of 
the pile-heads was on the 24th of April, 1860. The subsidence had 
then arrived at an average of 1 inch within an appreciable fraction, 
showing an increase in five years and five months of - 3 % of an inch. 
It is probable that this subsidence took place within the two or three 
years that followed the date of April 12, 1856, and that the piles 
have remained immovable for the last two years at least. 

The whole period over which this experiment extended was (May, 
1860) about nine and a half years. The conclusions that may be de- 
rived from it are obvious ; that the subsidence had ceased may be safely 
assumed. It follows that a building on piles, driven in soils exactly 
of the nature of that in which these experimental piles were placed, 
will be safe if we do not load them with a greater weight than the 
third part of the quotient arising from dividing the product of the weight 
of the ram and the distance it falls by the distance the pile is sunk by the 
last blow. The co- efficient was, however, fixed by Major Saunders for 
safety at one-eighth. Conversely, having given the weight of the su- 
perstructure, we can by the same rule ascertain the minimum number 



26 

of piles that will sustain it if driven to a fixed depth, or the depth 
to which an approximately fixed number of piles must be driven to 
effect the same purpose. 

There were two experiments of the kind we have described made at 
Fort Delaware. The second one continued three and a half years, had 
similar results, and was equally regarded in the determination of the 
co-efficient used in the rule. 

From other experiments made during the same period, Major San- 
ders ascertained the relation between the living force of the ram and 
the distance the pile is sunk for different falls by a series of experiments 
on 64 piles, which received 1,900 blows from a ram of 800 pounds. It 
was found that when the fall was less than 3 feet the useful effect was 
extremely small ; that it gained in a rapidly increasing ratio as the 
fall was augmented, a foot at a time, to 5 feet ; and that at this point 
the ratio of useful effect to the force expended is at its maximum, and 
that the piles are driven to distances proportional to the blow j or, in 
other words, that there is nothing gained by increasing the fall beyond 
5 feet ; for example, 2 blows of 5 feet will sink a pile as much as 1 blow 
of 10 feet ; 3 blows as much as 1 of 15 feet ; 4 blows of 5 feet as much 
as 1 of 20 feet. It was also found that if the 5-foot blows followed each 
other in rapid succession, the useful effect was rather greater than if 
the interval employed in common hand-power machines for hoisting 
the ram was allowed to elapse. 

From 1833 to 1838, eleven thousand (11,000) piles, of 45 feet in length, 
12 inches square at the head, and not less than 10 inches diameter at 
the small end, were driven for the foundations of Fort Delaware, under 
the superintendence of Captain Delafield, of the Corps of Engineers, 
with a hammer of 1,800 pounds, by blows in quick succession, with 
steam-power — the maximum fall of the hammer being 45 feet. Since 
1850, four thousand five hundred (4,500) additional piles were driven 
under Major Sanders' superintendence, from which experience he has 
drawn the preceding deductions. Six thousand six hundred of these 
piles are under the scarps and casemates of the present work. (See 
Lieut. Morton's Life and Services of Major John Sanders, of the Corps 
of Engineers, 1861.) 

The excavations for the foundations of the work existing at this 
time (1868) were commenced in 1848, and completed in 1849, and the 
piling by Major Sanders, heretofore referred to, was finished in 1851. 
By 1853, two thousand tons of masonry had been laid on these piled 
foundations, and in 1859 the walls, arches, and other masonry were 
finished. 



27 

Three bench-marks were established in 1854, reference to which was 
made in 1859, when it was fonnd the masonry had not settled in any 
part. 

From 1859 to October, 1866, the settlement, on reference to the above 
bench-marks, was 4 inches at the maximum point and 2".65 at the mini- 
mum, and a mean settlement for all the observed points of 3 /r .19. No 
crack was perceptible in any part of the work in 1866 or in 1868. (See 
Eeport of Superintendent at Fort Delaware.) 

RESISTANCE OF PILES DRIVEN AT THE SOUTHWEST PASS OF THE 
MISSISSIPPI RIVER, ON STAKE ISLAND. 

Stake Island consists of two mud-lumps, forming its northwest and 
southeast extremities, about 500 yards apart. At both points the 
ground is high, of the character of other mud-lumps. 

The space between the two mud-lumps is much lower, presenting the 
same general features and vegetation as the land on either side of the 
river. The ground between these two lumps is about as solid as any- 
where higher up on the river between the passes and the city. The 
experimental pile-driving commenced on the 19th November, 1867. 

Pile No. 1 was 30 feet long. After 12 blows the head of the pile, 
within 1' 6" of the ground, was rent to pieces. The head of the pile 
was square timber, 11 inches at the upper end and 10 inches diameter 
at the lower end, banded with J-inch iron 3 J inches wide; outside 
diameter, 9 J inches. The broken part of the pile was then cut off, and 
a piece 15 feet long set on top of it. Seventeen more blows were then 
given, when the pile gave way, tearing the head completely off. The 
total depth obtained by these 29 blows was 41' 11", being driven to 
low-water mark and level with the lowest soil. 

Pile No. 2, after 12 blows, was broken at the head, cut off, and a 
piece of 17 feet in length set upon it ; 23 additional blows drove the 
pile down to 45' 1" below the surface of the ground. 

Pile No. 3, 30 feet long. After 11 blows the head of the pile was 
badly rent 4' 6" above ground. Three feet 9 inches were cut off, and 
a piece of 18' 6'' set on, when with 9 blows the head gave way. Five 
feet 1 inch was then cut off, and with 7 more blows the pile was 35' 8" 
below the surface. 

Pile No. 4 was the same size as the previous piles. After 10 blows 
the head was V 8" above ground, and was then cut off, and 16 feet set 
on; 27 more blows were given, and V 9" cut off, when a piece of 16 
feet was set on, and after 13 more blows 4 feet were cut off, leaving the 
pile 54' 2" in the ground, and 5 inches above. 



'28 

The maximum fall of the hammer was 28 feet. The size of the ham- 
mer, of cast-iron, was about 2' 5" high, V 8" wide, and V 1" thick. 

The penetration of these piles was as follows : 

No. 1, with the first blow falling 5' 9", was 6' 3". 

No. 2, with the first blow falling V 6", was V 8". 

No. 3, with the first blow falling 4' 3", was 9' 1". 

No. 4, with the first blow falling 3' 6", was 11' 4". 

No. 1, with the last blow falling 28' 5", was 10 inches. 

No. 2, with the last blow falling 28' 7", was 11 inches. 

No. 3, with the last blow falling 26' 6'', was 9 inches. 

No. 4, with the last blow falling 25' 5'', was 14 inches. 

(See Bonzano to Gen. McAlester, 9th January, 1868.) 

In March, April, and May, 1868, 45 piles, of 50 to 60 feet in length, 

were driven under the direction of General McAlester, at St. Joseph's 

Island, in the same alluvial soil as deposited from the Mississippi river. 

The same remarkable uniformity of penetration, by the successive blows, 

was developed as at Stake Island. (See McAlester's report of 5th May, 

1868.) 

GRILLAGE FOUNDATIONS. 

A third system of foundations for compressible soils is that of the 
grillage, resting on the natural formation on the surface or at any 
selected depth. 

The following are examples of this character by engineers who have 
constructed many edifices on this principle in the United States : 

The earliest structure of this character, of which the writer has in- 
formation, is a building at the Balize Bayou of the Mississippi river, 
near the site of the pilot-houses. It is of brick masonry, constructed 
by the Spaniards or French during the early settlement of the country, 
and apparently for a magazine. It had settled so deep, up to the year 
1829, as to make it impracticable, at that time, to ascertain the uses 
to which it may have been applied. From the practice of the inhabit- 
ants of the country it is inferred, in the absence of positive knowledge, 
that a grillage was used in this case. 

In 1807, a survey of the coast of the Mississippi had been made, and 
the outlets of the Mississippi particularly examined to select the most 
eligible site, best material, and plan for a light-house. A commission, 
appointed by the President of the United States, in 1816, again explored 
the various mouths of the river and the different islands situated there, 
and concluded that the most proper situation was at the mouth of the 
Northeast Pass of the Mississippi river, on Frank's Island. They say 
"this site appears to have undergone all the changes experienced by 



29 

the different Islands here in the course of their formation and consolida- 
tion, being elevated about 3 feet above the surface of the river, and 
the only island not covered with water during the last hurricane." They 
bored the island and found the soil, for 35 feet, a mixture of clay and 
fine sand, and to the depth of 50 feet, a dark blue clay without any 
mixture of sand or vegetable matter. This clay grew harder as the 
borings descended. This commission designed a plan for this locality, 
the structure to be built of stone and brick, recommending that, for a 
foundation, the surface to be covered by a light-house, be dug down to 
the level of low water, and this space be filled with piles 25 feet long, one 
foot diameter, driven as close as possible, and as long as they can be 
forced down with the "battering ram, 1 '' then cut off at the surface of the 
water, laying upon their heads one foot square timber of the greatest 
length that could possibly be procured, and not more than V 6" apart, 
crossing these with another layer of the same dimensions, filling the 
intervals between the timbers with shells, or rubbish, beaten down and 
united by pouring in grout; covering this with a close floor of 4-inch 
plank, spiked to the timbers. Upon the floor the foundations may be 
laid, taking the precaution to turn reversed arches under all the walls, 
the weight to be diminished comparatively by making it bear on the 
greatest surface possible. (See State Papers on Commerce and Agri" 
culture.) 

A light-house of stone and brick masonry was afterwards built at 
this locality. After being finished it soon settled irregularly, and 
finally inclined to one side so much as to make it necessary to take 
down the whole structure, before the year 1824, soon after which period 
the material was in part transported to and used in the construction 
of Fort Jackson, about thirty miles up the river. By reference to the 
original drawings of Mr. Latrobe, (who constructed the building,) and 
loaned me by his brother, Benj. H. Latrobe, of Baltimore, it appears 
that the masonry of this tower was 81 J feet high, and 19 feet diameter 
at the base, resting on 53 piles of ten feet in length. 

The foundations of Fort Pike, on the Eigolets, were commenced about 
the year 1820. The soil is very similar to that at the mouths of the 
Mississippi, the surface being mostly vegetable matter underlaid by a 
combination of vegetable earths and dark clay. The foundations of 
the scarps and piers of the casemates were all laid on a grillage of 
round pine logs, side by side, resting on the excavated soil, and crossed 
on top at right angles, by similar logs, of from 10 to 12 inches diameter, 
and about two feet apart in the clear. The masonry of the piers was 
built in and thus united to the masonry of the scarps. The work was 



30 

finished about the year 1828. The foundations are about six feet be- 
low the level of the waters of Lake Pontchartrain, and the ground be- 
low that level is saturated with water. 

The work settled irregularly. The masonry of the casemate piers 
broke from the scarps, making cracks about 4 inches wide. The timber 
grillage in this case failed to overcome the compressibility of the sub- 
soil, or to preserve uniformity of settlement. (Personal recollection.) 

Fort Wood, on the Ghef Menteur Pass, La., (now, 1868, Fort Ma- 
comb,) was commenced about the year 1825 and finished in 1832. It is 
situated at the end of a dry clay ridge following the bank of the Me- 
tarie Bayou, from near New Orleans to the entrance of this bayou into 
Pass Chef Menteur, and is the firmest soil on which any of the forts in 
Louisiana are constructed, the natural surface being three feet above 
the waters of the Pass and Lakes Pontchartrain and Borgne. This dry 
ridge is wooded with heavy live-oak timber. The foundations of the 
scarps and casemates are similar to those of the fort on the Eigolets, 
and the general plan of the work similar. It was finished about the 
year 1832. The foundations were about six feet below water, and the 
ground saturated from the lake level to the depth of the excavations. 
The excavations were in a firm, adhesive clay soil j hand-pumps and 
buckets sufficed to keep down the water during the excavations. 

In this case there was no such irregularity of subsidence as to injure 
the work, although extensive repairs were necessary in consequence of 
this irregularity. The grillage did not suffice to overcome the com- 
pressibility of the subsoil, or preserve uniformity in the levels of the 
masonry. (Personal recollection.) 

The foundations of Fort Jackson, Plaquemine bend of the Mississippi 
river, were commenced in 1825 ; the masonry was finished and ram- 
parts formed, ready to receive the parapets, in 1832, at which time the 
work was suspended to give time for settlement. The subsoil in this 
case is very compressible, and saturated with water at all times from 
near the surface to and below the bottom of the foundations, about 11 
feet below the natural level of the country. Vegetable matter com- 
posed the upper layers, under which it is combined with dark clay. 
The problem in this case was to create uniformity of pressure and set- 
tlement by underlaying the masonry with a strong grillage of timber 
laid on a plank floor. The plank was laid on the excavated earth and 
large cypress timber, 12 inches thick, 15 to 24 inches wide, laid longi- 
tudinally upon it, edge and edge, formed a solid continuous floor. On 
the top of this solid flooring, other timbers, 12 inches thick, and from 
15 to 18 inches wide, were laid 3 feet from centres, perpendicular to the 






31 

lower courses. The spaces between these last timbers were filled solid 
with brick masonry laid in cement mortar. This grillage projected 
three feet in front and rear of the fair masonry of the scarps, which 
were from eight to ten feet thick. The grillage and foundations of the 
casemates were constructed in like manner. The result, seven years 
after the commencement of the foundations, was unequal settlement of 
various parts of the work, and cracks in the masonry of the scarps on 
the faces and flanks of the bastions. It became necessary to lighten 
the load of embankment pressing on the foundations. The earth press- 
ing against the centre of the scarps of the bastion faces and of the solid 
curtains was removed to the depth of the grillage of the casemates, and 
hollow vertical brick cylinders of 18 feet diameter, covered with hemi- 
spheres of brick masonry, were constructed behind the faces of the 
bastions, and three horizontal bomb-proof store-rooms were built on the 
centres of the solid curtains. 

No stronger or more substantial grillage is known to have been con- 
structed in Louisiana. In 1842, the casemates of the two water-fronts 
and gate-way fronts were a second time loaded with an excess of weight 
to restore the levels and guard against future inequality of settlement. 
In 1851, the floors of the casemates of the flanks had to be raised and 
other repairs made, in consequence of unequal settlement. 

The grillage failed to preserve uniformity of settlement, and the com- 
pression of the subsoil was not overcome by this application of a 
timber platform distributing the weight over a great surface. (Per- 
sonal knowledge.) 

Battery Bienvenue, on the bayou of this name, emptying into Lake 
Eorgne, was commenced about the year 1828. It is founded upon a 
grillage of round pine logs, averaging 12 inches diameter, laid on a 
plank floor, and overlaid by 12-inch pine logs, at right angles to the 
lower layer, and distant about three feet from centres. This grillage 
is founded four feet below the surface of the water of the bayou. The 
soil is but little better than a prairie tremblante. Borings were made 
twenty feet deep, indicating vegetable soil only, combined with a large 
percentage of water. The grillage is arranged precisely similar to 
those of Chef Menteur and the Eigolets. The masonry consists of a 
low scarp, backed by an earthen rampart. The masonry settled sev- 
eral feet before the scarp was finished, with great irregularities ; the 
centre of each line of the trace of the work settling some feet below the 
ends. A square magazine, built on an independent grillage, similar to 
that under the scarps, settled soon after the bomb-proof arch was 
turned, several feet, bringing the lintel of the doorway down to the 






32 

surface of the ground. In this case the grillage failed to either secure 
a uniformity of settlement or overcome the compressibility even to such 
an extent as to fulfil the desired useful purpose. Great and extensive 
repairs and additions have been since made to this work. (Personal 
recollection.) 

Southwest Pass Light-house was built in 1831. It is about 65 feet 
high, and built of wood. It rests on a timber grillage of unknown 
dimensions. It has failed to secure the conditions necessary for a 
foundation and has now to be rebuilt, having inclined so far from the 
vertical as to render a hastened reconstruction desirable. The site of 
this light-house is on one of the upheaved islands common to the mouth 
of this river. (Records of the Light-house Board.) 

The St. Charles Hotel, in the city of New Orleans, was commenced 
in 1836 and finished in 1837. It was destroyed by fire in 1851. The 
foundations were of brick masonry, on a grillage of heavy flat-boat 
gunwales of 60 to 80 feet in length, 20 to 30 inches wide, and 6 to 12 
inches thick, and laid about 6 feet below the sidewalk. This building 
settled 2 feet in 12 or 14 years. The present hotel (1868) was com- 
menced in 1851, on top of the old foundations, and finished in 1853. 
It has settled fully 12 inches more (in addition to the settlement of the 
former building) in the last 15 years, making a total compressibility of 
the subsoil of 3 feet since 1837, or in 31 years. In this case a grillage 
has failed to secure the desired object. (Henry Howard to Gen. Dela- 
field, June, 1868.) 

The Light-house at Pass a Loutre was built in 1855. It is a cast-iron 
shell, about 65 feet high, resting upon a masonry base 4' 6" thick, ex- 
tending a like distance (4' 6") beyoud the base of the tower and under 
the whole structure. In 1864, an interior brick lining was added. It 
is not known that any timber underlays the masonry. This masonry, 
however, covering the entire surface occupied by the building, with 4' 
6" offsets beyond it, fulfils all the conditions of underlying timber s. 
The site on which it is built is firm on the surface and is about 3 feet 
above the level of the Gulf of Mexico. It must, therefore, be upon one 
of the upheaved islands common at the outlets of the Mississippi. It 
has settled unequally, one side leaning from 9 to 12 inches in its height. 

In this case the grillage or masonry base has failed to secure either 
uniformity of settlement or to overcome the compressibility. The 
keeper's house at this locality is a lj-story wooden structure, resting 
on 9 brick piers of 18 inches square each, under which is a grillage 6 
feet square, of two thicknesses of round logs. It has settled 2 J feet 
and now requires repairs (1868) to correct the consequent defects. The 



33 

grillage has, in this case, failed to secure the desired stability. (Bon- 
zano's report to the Light-house Board, Washington.) 

The First Presbyterian Church, in New Orleans, was commenced in 
1846 and finished in 1857. The main tower is 20 feet square and 115 
feet high, surmounted with a wooden spire. The foundations were 
built of brick masonry, laid in cement mortar, on a grillage of two thick- 
nesses of flat-boat gunwales of 45 feet in length, crossing one another. 
The excavation was 12 feet below the surface of the street. The bot- 
tom was a hard, stiff, blue clay, the best my informant has ever seen, 
after thirty years' practice as an architect in New Orleans. It has 
settled 5J inches in 11 years. (Henry Howard to Gen. Delafield, June, 
1868.) 

Fort Calhoun, now Fort Wool, Hampton Eoads, Va., was commenced 
in April, 1819, when the first stone was deposited on the Eip-rap shoal 
for its foundation, in 21 feet water. The shoal and adjacent shores of 
the Chesapeake bay and Hampton Eoads are hard sand. It was deter- 
mined to construct the superstructure of a four-tier casemated masonry 
battery at this locality, on an enrockment of quarry stone, thus forming 
an island similar to the Plymouth and Cherbourg breakwaters. Dur- 
ing the first year's labor the enrockment was brought to the surface of 
the water on the line of the southern face of the work, and in 1824 the 
surface had been enlarged to receive the entire foundations of the super- 
structure, for which preparations were made and commenced in 1826. 
The enrockment was added to, and an area of several acres was formed, 
terminating with a long slope, to intersect the shoal — equivalent to a 
grillage for the proposed superstructure, spreading the pressure of the 
walls at least 25 feet beyond their exterior lines. The foundations of 
the casemates on the water fronts were all laid, and the scarps and 
piers of the casemates raised to the springing lines of the arches of the 
embrasures, when numerous cracks in the masonry and irregular set- 
tlement of the work throughout its whole extent indicated that the 
foundations could not be relied upon. It was determined to suspend 
the construction of the work in 1829 or 1830, and load the foundations 
of the scarps and casemates with a weight greater than that of the 
castle in its finished state, with its armament, stores, and garrison. 

In 1833, 11,000 tons of building- stone were piled over and near the 
walls, bearing on their foundations. 

In 1834, 61,866 tons were added. The annual subsidence with this 
increased weight at the centre of the work was one-third less than it 
was in 1833, and those parts that formerly settled most, now settled 
least. 

3 M 



34 

In 1835, 20,000 tons more than the estimated required quantity was 
resting upon and along the whole extent of the foundations. This acces- 
sion of weight continued to cause subsidence, though in a decreasing 
ratio. 

In July, 1836, the stone for the superstructure and materials ac- 
cumulated for compressing the foundations were being removed, upon 
the supposition that the base on which the castle was to rest had been 
satisfactorily compressed. In 1837, the load of stone was entirely re- 
moved from the foundations ; continued subsidence was, however, ob- 
served, and the foundations were immediately reloaded. 

In 1840, a total weight of 55,716 tons had been reloaded on the 
foundations — increased subsidence was still observed. In November 
1842, there was an excess of 13,627 tons on the foundations, beyond the 
ultimate weight to be supported. The average subsidence in 1841 was 
seven-eighths of an inch; compared with former years it was in a de- 
creasing ratio. In 1843, it was found that the mass was still settling, 
when an additional load was recommended. In 1846, the subsidence 
was three-fourths of an inch, and up to 1850 the diminution of settle- 
ment was progressive. 

The total average subsidence of the foundations from 1830 to 1856, 
accurately noted from year to year, was at the greatest point 5.28 feet, 
and at the least 4.35 feet. It was immovable from 1850 to 1856. From 
1841 to 1847, the maximum* average settlement was T fo of a foot. The 
total subsidence of a tide pier from 1824 to 1851 was 6.63 feet. (See 
Annual Reports of Chief Engineers, accompanying President's Mes- 
sage.) 

Custom-house, New Orleans, La. This building was commenced in 
1848, and progressed from time to time until 1860. It is founded upon 
a flooring of plank laid on the excavation seven feet below the street 
pavement. On this floor a timber grillage is laid of 12-inch logs, 
"side and side," over which similar logs are placed transversely, dis- 
tant from each other two to three feet in the clear. 

The space between the timbers is filled with concrete, which is con- 
tinued over the whole grillage for a depth of one foot. Counter arches 
of 1^-bricks thick support the walls of the interior subdivisions of the 
building, thus throwing the weight of the building upon the entire 
surface included within the outline of the building. 

As a concrete and grillage foundation to support this heavy building, 
no greater surface, to resist the weight and pressure of the walls, 
could well be attained. 

The walls, of 2' 6" thick, rest on grillage timbers 10 feet wide. 



35 

Those of 4 feet thick, rest on grillage timbers 15 feet wide. 

And those of 9 feet thick, rest on grillage timbers 20 feet wide, 
(abutments.) 

In 1860, the granite walls of this building had been carried up 75 feet 
above the concrete base to the architrave line of the entablature, and 
all the iron floor-beams of the fourth story finished. 

The exterior walls are 4 feet thick, exclusive of projections ; 2J- feet 
of which is brick masonry and 1J feet of stone masonry. 

In 1851, a commission reported that borings at the custom-house, and 
at a point not far removed from it, indicated different degrees of com- 
pressibility. It is situated upon the firmest, dryest, and most reliable 
ground in and about the city of New Orleans. 

The maximum settlement of the building in any one point up to 1860 
was 2' 6". This compressibility of the soil beneath the grillage was 
very variable. 

From 1848 to 1851, the maximum settlement was 22.57 inches, and 
the minimum in the same time was 15.63 ; making a difference of level 
in various parts of the building of 6.94 inches. 

In the year 1857-'58, the maximum settlement was 3.50 inches and 
the minimum 0.66; making a difference of settlement this year of 2.84 
inches ; and in 1858-'59, the maximum was 2.63 inches, and in many 
places nothing; making a difference of settlement, level, and com- 
pressibility this year of 2.63 inches. 

The line of the exterior walls in 1864, on which the temporary roof 
rests, varies in the level 3 inches. 

This grillage covers a surface of about 300 feet square, and although 
well constructed and with great care, it has failed in its objects. (See 
Eecords of the Treasury Department, Bureau of the Architect.) 

Fort Sumter, Harbor of Charleston, was constructed upon an enrock- 
ment, similar to Fort Calhoun, now Fort Wool. In 1840, the settle- 
ment was such as to induce the superintending engineer to recommend 
that it be loaded with a weight equal to the maximum, with its arma- 
ment and munitions that could rest upon its foundations. In 1850, the 
engineers reported the subsidence of this work to be continuous, 
though in a decreasing ratio annually. At this time the scarps and two 
tiers of casemate arches were completed. 

CONCLUSION. 

The plan proposed by a committee of the Light-house Board, in 1868, 
is again referred to the same committee, increased by two other mem- 
bers of the board, to consider the whole subject anew. The plan then 



36 

approved by the board is appended hereto ; which, with the practice 
and views of many experienced engineers in compressible soils, is pre- 
sented, with the belief that the best plan suited to this most difficult 
locality, may be more satisfactorily devised with the information now 
embodied in this memoir for the consideration of the committee. 

Extracts: " Considering the impracticability of finding any solid 
natural base at the outlets of the Mississippi, or any locality calculated 
to sustain the weight of an iron light-house, without extraordinary aids 
from art; and the extraordinary geological formation of the country 
about the Passes of the Mississippi, all going to show that great and 
unusual precautions are necessary to secure a foundation for any weighty 
structure at or about the outlets of the Mississippi, the committee pro- 
poses some modifications of the plans submitted to it, with the hope of se- 
curing greater solidity, and to a, greater depth of the natural soil to 
resist the settling of an iron light-house for the proposed locality at the 
Southwest? Pass." 

"The first precaution is, that the superstructure to rest on this foun- 
dation shall be of the least practicable weight to fulfil all the condi- 
tions for a permanent light-house, and then that the soil of the selected 
site be solidified to the greatest practicable depth by wedging it with 
wooden (pine) piles of about 50 feet in length, not less than 12 inches 
square at the head, and not less than 10 inches diameter at the lower 
end, driven into the soil with a hammer of not less than 1,800 lbs. 
weight, with a final fall of 45 feet, continuing the operation of driving 
until the head of the pile may pass below the ways of the pile-engine, 
and thence by a punch to the surface of the soil, or until the head of the 
pile, battered into fibres, no longer transmits the percussive action of 
the ram to the solid wood of the pile. These piles should be driven in 
rows, 3' 6" from centres throughout the entire surface of the site to be 
occupied. After driving this series of piles, a judgment can be formed 
of the practicability of giving additional solidity to the natural soil by 
driving another series of piles of the same dimensions (or somewhat 
smaller diameter) at the intersection of the diagonal lines drawn from 
the heads of four consecutive piles of the first series, and 3' 6" from 
their own centres in their own parallel lines or roads." 

"No more timber than of these two series can probably be driven into 
the soil. Should the contrary be the case, the site should be abandoned 
as approaching too near a semi-fluid to justify the construction of any 
permanent structure thereon." 

"Having thus solidified the soil by wedging with timber, the heads 
of all the piles of the first series are to be cut off level, to a uniform 



37 

depth of 2' &' below the lowest low water, and the heads of the second 
series to a uniform level of V 6" below the lowest low water." 

"The next operation will be to secure the heads of all those piles in 
a fixed position in relation to each other, and from being inclined by 
the superincumbent weight of the light-house. This will be effected by 
excavating the soil to a depth of V &' below the heads of the first series 
of piles cut to a uniform level as above required j (it may be found more 
advantageous to make this excavation before driving any piles, to ad- 
mit of floating the pile-engine on a scow, a great economy where prac- 
ticable;) or, to a depth of four feet below the lowest low water." 

" The space below and between the heads of the first series of piles 
to the depth of 1/ 6" will then be filled with concrete to the heads, care 
being taken to pump out all the water, that the concrete can be rammed 
thoroughly between the piles and into the soil on which it is first 
deposited. This concrete will be brought to the level of the heads of 
this series of piles, thus bracing them from each other by a hard, 
durable, and continuous mass of concrete of V 6" thick over the whole 
surface of the foundation." 

" Over the heads of this first series of piles, and resting also upon 
the concrete surface between them, horizontal timbers (cypress or pine) 
will be laid, 12 inches thick by 15 to 18 inches wide, when another layer 
of concrete will be rammed about these string-pieces and the heads of 
the second series of piles ; then a row of horizontal 12-inch timbers at 
right angles with the first, of 15 to 18 inches wide, will be laid over 
the heads of the second series of piles, and concrete filled in solid to 
the surface of the second layer of horizontal-bearing pieces. The string- 
pieces resting on the heads of the piles may be treenailed thereto to keep 
them in place while the work is being constructed." 

u We have now solidified the soil to a depth of fifty feet, and above 
it formed a solid floor of timber and concrete, resting upon and cover- 
ing the entire base thus solidified by wedging." 

" Upon this last surface, which is six inches below the lowest low 
water, completely excluded from the air, the foundation of the masonry 
for the support of the iron of the superstructure will be commenced." 

The foundation of the base of the iron-work should be four feet above 
ordinary high water. " This foundation, down to the level of the tim- 
ber-grillage, should be formed of solid brick or stone masonry, spread- 
ing out to cover the greatest possible surface, by offsets in every 
course of brick or stone of which this part may be constructed. Be- 
tween this exterior base and the centre, the entire foundation will be 
raised with concrete to the natural level of the shoal. As these parts 



38 

cannot have any weight of the superstructure (of the proposed design ) 
upon them, any superfluous mass of material but adds to the risk of 
settling." 

" The superstructure should be designed to throw the weight not 
only on the corner-posts, but upon the spaces from one of these corners 
to the other, and also upon the radii connecting these corners with the 
centre column. With this in view, the masonry on these lines, from 
the timber platform, should be built with offsets to cover as much sur- 
face as practicable. The bed -pi ate of the superstructure resting on the 
masonry should be bolted with composition bolts, securing them to the 
under part of the concrete and timber platform, to give some additional 
stability to the base against the action of the wind." 

" Notwithstanding all these precautions, it is considered advisable, 
previously to raising the superstructure, to load the entire surface of 
this artificial foundation with a weight of matter greater than the 
entire load it can ever be required to sustain. This load should remain 
at least one year, when an examination should be made as to the pro- 
priety of commencing the superstructure, suspending the work, add- 
ing to the load and awaiting another year, or abandoning the site 
altogether." 

The weight of cast and wrought-iron of the superstructure of the 
plan submitted to the committee is 400,000 pounds. This may be 
greatly reduced by the adoption of modification of the design of the 
framing of the wrought-iron work. Eeference to the Practice of Euro- 
pean Engineers for such structures may be found in Proceedings of the 
Institution of Mechanical Engineers for 1861, giving a detailed descrip- 
tion of the light-house at Buda, Spain. Minutes of Proceedings of the 
Institution of Civil Engineers, vol. 23, for a description of the Ushruffee 
light-house, in the Eed Sea, and the Engineers' and Architects' Journal 
for January 7, 1868, for description of the light-house for the Douvres. 

EICHAED DELAFIELD, 

Brevet Major- General, Corps of Engineers, U. 8. Army. 






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